1. Field of the Invention
The present invention relates generally to surface-condition sensors.
2. Description of the Related Art
The presence of water and ice on roads has been the primary cause of an enormous number of road accidents. Many of these accidents could have been prevented if these dangerous conditions were sensed and knowledge of them promptly communicated to approaching drivers. Accordingly, a successful road monitoring system would return significant rewards to the public in the form of injury and death prevention and economic savings.
Considerable efforts have been directed to the development of a practical road sensing system. For example, Intelligent Vehicle Highway System (IVHS) is a long term program funded in the United States at approximately 660 million dollars. This program includes the development of Advanced Traveler Information Systems (ATIS) which require sensors for monitoring of road conditions.
Road condition sensors must operate in the harsh environment of a road surface. Consequently, they preferably are simple, easily installed, resistant to the pressures and abrasions induced by vehicle fires, relatively unaffected by environmental effects (e.g., heat, cold, rain and ice) and reliable (e.g., having a low false-alarm rate).
A large number of systems have been proposed for detecting road surface conditions. One exemplary system positions a series of exposed electrodes on a road surface to measure electrical conductivity, ionic polarizability, stray capacitance and roadbed temperatures at different depths. These multiple measurements are integrated and analyzed to determine the presence of surface coatings, e.g., frost, ice, water and so on.
Another exemplary system positions a capacitance bridge in a sensor disk which is embedded in a road surface. Closely spaced conductors are also embedded in the surface for measurement of the resistance of road coatings and a thermistor is mounted below the road surface to detect whether the temperature is above or below freezing. The output of these sensors is combined and analyzed to determine the presence of different coatings on the road surface, e.g., the output of the capacitance bridge is used for detection of a water coating.
Although these systems may successfully sense road conditions, they require the installation of multiple components, would typically be sensitive to wear and abrasion, and involve the measurement and integration of several parameters.
Other ice-detection systems have proposed the use of microwave signals. In copending U.S. patent application Ser. No. 08/324,436 (filed Oct. 17, 1994 and assigned to Hughes Aircraft Company, the assignee of the present invention), now issued as U.S. Pat. No. 5,497,100, with issue date Mar. 5, 1996, assigned to Hughes Aircraft Company a microwave signal is directed at a road surface to generate a reflected signal. The reflected signal contains information which is indicative of dielectric coatings, e.g., water and ice, on the road surface. This system requires the generation of very high frequencies, e.g., millimeter waves, and is primarily intended for use on a moving vehicle.